1. Field of the Invention
This invention relates to phase shift masks, and to phase shift mask blanks as base materials therefor, wherewith the resolution of the transfer pattern can be enhanced by imparting a phase difference between the exposure light beams passing through the masks, and more particularly relates to so-called halftone phase shift masks and halftone phase shift mask blanks.
2. Description of the Related Art
In the manufacture of semiconductor LSIs, photo-masks are used as masks during fine pattern exposure. One type of such a photo-mask that is used is a phase shift mask made so that, by imparting phase differences between the exposure light passing through the mask, the resolution of the transfer pattern can be enhanced.
In recent years, the halftone phase shift mask has been developed as one type of such a phase shift mask, and has come into use.
One such halftone phase shift mask that is known in the prior art is the halftone phase shift mask disclosed in Unexamined Patent Application No. H6-332152/1994, in gazette, which is particularly well suited to the transfer of patterns wherein single holes, dots, and lines are independent.
The halftone phase shift mask disclosed in the cited application configures a mask pattern formed on a transparent substrate with a light transmitting portion that transmits light of an intensity that contributes substantially to exposure and a light translucent portion that transmits light of an intensity that does not contribute substantially to exposure. The phase of the light passing through the light translucent portion is shifted, so that the phase of the light passing through the light translucent portion bears a relationship respecting the phase of the light passing through the light transmitting portion that is substantially inverted, whereby the light passing in the vicinity of the boundary between the light transmitting portion and the translucent portion cancel each other out, thus making it possible to secure good contrast at the boundary.
In such a halftone phase shift mask as this, the light translucent portion has two functions, namely a light blocking function for substantially blocking the exposure light and a phase shifting function for shifting the phase of the light. Thus it is not necessary to form a light blocking film pattern and a phase shift film pattern separately, wherefore the structure is simple and fabrication is simple.
One known conventional example of a halftone phase mask blank for use as the material in such a halftone phase mask is diagrammed in FIG. 11 and 12.
The halftone phase mask blank diagrammed in FIG. 11(a) has a molybdenum silicon (MoSi)-based halftone material film 2, for example, formed on a transparent substrate 1. The halftone phase mask blank diagrammed in FIG. 12(a) has a molybdenum silicon-based halftone material film 2 formed on a transparent substrate 1, but also has, on top of that halftone material film 2, a molybdenum metal film 3 for the purpose of preventing the transparent substrate 1 from being charged during electron beam exposure so that the path of electron beam advance becomes unstable.
In order to make halftone phase masks from these halftone phase mask blanks, as indicated in FIG. 11(b) and FIG. 12(b), a resist film 4 is first formed on the respective halftone material films 2. Then, after subjecting this resist film 4 to electron beam exposure and developing, the desired resist patterns 5 are formed, as indicated in FIG. 11(c) and FIG. 12(c). However, when the molydenum metal film 3 and/or the halftone material film 2 is etched using this resist pattern 5 as the mask, the resist pattern 5 itself gets etched, beginning from its edges. Therefore the dimensions of the halftone material film 2 cannot be controlled accurately, as a result of which it is not possible to etch the halftone material film 2 with high precision, which is a problem.
Furthermore, in the process steps from film formation to the point where the halftone phase shift mask is finished, various chemicals such as acids and bases are sometimes used. Thus a problem arises when, due to the use of such chemicals, the properties of the halftone material film 2 are changed, and the desired halftone properties are not obtained.
In order to resolve such problems as these, a halftone phase shift mask blank is proposed in Unexamined Patent Application H8-101493/1996, in gazette, wherein a metal film 6 that enables the selective etching of the halftone material film 2 is formed on that halftone material film 2, as diagrammed in FIG. 13(a).
This halftone phase shift mask blank is configured with a MoSiOxNy (where x.times.y=an integer) halftone material film 2 formed on the transparent substrate 1, and a Cr metal film 6 laminated on the halftone material film 2.
By configuring the phase shift mask blank in this way, the Cr metal film 6 and MoSiOxNy halftone material film 2 can be etched independently, making it possible to subject the halftone material film 2 to various desirable treatments, and thus making it possible to pattern the halftone material film 2 with high precision.
In the halftone phase shift blank diagrammed in FIG. 13(a), however, when the halftone phase shift mask is being fabricated, a resist film 7 is formed on the metal film 6, the resist film 7 is subjected to electron beam exposure, as diagrammed in FIG. 13(b), and is then developed, forming the desired resist pattern 8 diagrammed in FIG. 13(c). When the metal film 6 or the metal film 6 and the halftone material film 2 are etched, using this resist pattern 8 as a mask, the metal in the metal film 6 remains on the halftone material film 2, whereupon the halftone material film 2 is patterned with that fine metal acting as a mask, resulting in flaws developing in the pattern, which is a problem.
Also, when the Cr metal film capable of selecting etching is formed on the MoSiOxNy halftone material film 2 and this Cr metal film 6 is patterned by wet etching, the Cr metal film 6 etching rate is slow. While it cannot be said that this has absolutely no effect on the MoSiOxNy halftone material film 2, it does have considerable effect on the optical characteristics.
Furthermore, the bonding strength between the halftone material film 2 and Cr metal film 6 is inadequate, leading to a problem of film peeling.